GL5307 Introduction to Petrophysics
MSc Program: Reservoir Engineering - University of Aberdeen, UK
Assessment - Deterministic Interpretation
For this assessment you will perform a number of calculations within the provided excel spreadsheet and use these data to answer the questions in this ppt.
Background: You have already undertaken a short quick look analyses of the A2_Logs 1_GL5307 file as a class - refer to Practical 4 ppt. for background
Through this process you should already have completed the following steps:
1. Calculate Vshale using the gamma ray log and values of Grsa = 20 and GRsh = 90.
2. Apply a sand cut-off of < 40% Vsh to define potential reservoir intervals (in excel you can use a logic function: = IF(Vsh < 0.4,(1),(0)). This will give a simple 1 for reservoir and 0 for non-reservoir.
3. Assess fluids within the reservoir rocks combining the N-D and resistivity logs, where would you pick the OWC?
4. Calculate density porosity assuming a fluid density of 1 g/cm3 and 2.65 g/cm3 initially for a sandstone matrix.
4. Now set the porosity to zero for all values of Vsh above 40% =IF(Vsh<0.4,(Porosity),(0)).
These steps do not form part of the assessment.
The following workflow forms part of Assessment:
1. Using the Rwa technique, calculate the resistivity of the formation brine in the water leg at 649.07 m depth.
2. Name a common alternative method of calculating Rw based on a cross plot of Rt versus porosity in the water leg?
3. Using this value of Rwa, calculate the water saturation (of the reservoir intervals only) using the Archie equation. Assume that a=1 and m=n=2 for now. Based on your working, what is the Sw value at: 642.06 m? 635.05 m? 628.04 m?
4. Replace the blank log to the right of this slide with the plot provided in the excel worksheet showing your calculated values. What would you term the interval between c. 646 and 640 m based on the Sw?
5. Given that fluid density affects the density porosity calculation, modify the fluid density to 0.9 g/cm3 for the oil leg (calculate density again in 'Dens (water/oil)' column provided) and re-calculate Sw in the provided Sw2 column in excel. What effect does this have on the Sw in comparison to before at: 642.06 m? 635.05 m? 628.04 m?
6. Does your choice of fluid for the density porosity calculation have the potential to influence reserves calculations?
7. The results of coring are back from the lab and you now have a core based lithology determination. The coring highlighted a number of limestone units in the well. Using the neutron-density logs, what is the average porosity of the limestone encountered between 625.145 to 626.516 m depth? Give the average porosity assuming a matrix density of 2.7 g/cm3 and fluid density of 0.9 g/cm3?
8. Can you suggest a reason for the lowering of porosity and increase in resistivity from 650 to 655 m?
9. Can we reliably use the neutron and density logs to derive effective porosity in the shales? State one reason?
10. No clear difference in the degree of Neutron-Density separation is seen between the oil and water legs. Would you expect the separation to increase in the oil leg with increasing porosity?
11. Based on your updated interpretations so far, fill out the table (attached).
Use the porosity values in the 'Dens. por (<40% sh)2' column to calculate Average porosity (exclude non-reservoir and any zero porosity intervals for now)
Use the Sw values in the 'Sw2b' column to calculate average Sw (this column sets a maximum Sw of 1 based on your input data). As with porosity, exclude non-reservoir and any zero porosity intervals for now.
12. Another useful way of highlighting the distribution of hydrocarbons based on the wireline data is to calculate a ShPOR column using the equation: ShPOR= (1-Sw)*Porosity. Fill in the ShPOR column and replace the associated plot to the right of this slide with your calculated curve.
You have now been provided with formation pressure test data for the reservoir (see Tab 2 of the spreadsheet). Refer to your lecture notes on Formation Testing to help you with this section.
You are provided with a set of measured pressures (in psi), along with the depths at which the test were taken (remember you are working in meters for this example).
13. A partially completed graph of Depth versus pressure is available. Add the remaining data points to the graph as a separate series and define the parameters c (intercept of regression line at P=0 (m)); and m (slope of the straight line regression equation (m/psi) for the deeper set of 3 samples?
14. Copy your cross-plot with the line equations displayed into this ppt page, replacing the partially complete example.
15. Based on these values, complete the column 'Model depth'. This gives a set of values that fall perfectly on the straight line model for the given pressures (remember you have two different straight line models).
16. Now fill out the missing parameters in the provided table using the 'Fpress' and 'Model depth' data columns and copy the completed table into this ppt. slide. Note, when calculating the density, you will need to convert G from psi/m to psi/ft e.g. use: ρfluid = (0.3048x G)/0.433.
17. Based on your results, can you confirm the presence of hydrocarbons in your reservoir? What kind, oil or gas?
18. Finally, calculate the depth of the FWL (answer to two decimal places) based on the two gradients you have characterized (equations are given in the spreadsheet). Replace the cells copied at the top of this slide with your calculated results.
19. How does this depth compare with your previously interpreted OWC. The closer they are, the more likely the reservoir sands are of good quality, do you interpret good or poor quality sands?
Finally, you receive a sub-set of RCA and SCAL analyses from the lab. Use these data to assess the following:
20. Using the overburden SCAL data for porosity, calculate the fractional decrease in porosity for all the samples using the 50 psi data as the base-line e.g. calculate porosityOB/porosity(50 psi), for the four available samples.
21. Plot these results against the overburden pressure for all four sample series. Which of the available excel trendlines gives the highest R2 value (use an average value calculated at each pressure to test the fit)?
22. Assuming a lithostatic gradient of c. 1 psi/ft, choose the isostatic pressure measurements closest to your depth of interest for this reservoir (you could extrapolate the exact depth from the modelled line, but for now, just choose the closest experimental pressure increment)?
Attachment:- Introduction to Petrophysics Assignment Files.rar